1. Field of the Invention
The present invention generally relates to a vernier type motor having permanent magnets placed on a rotor thereof, and more particularly to an improved rotor structure of the vernier type motor.
2. Description of the Related Art
FIG. 11 shows a relationship between a stator and a rotor of a conventional vernier type motor having permanent magnets placed on the rotor. Numeral 1 indicates the stator and marks from S1 to S18 indicate slots. A bipolar three-phase alternating current winding, commonly used for such an induction motor, is looped through each slot. Teeth of the stator are formed so that the width of each front end is almost equal to the width of each of the inlets of the slots. Therefore, magnetic reluctance noticeably changes at regular intervals in a direction of stator rotation viewed from the rotor. Numeral 2 indicates the rotor and 17 poles of permanent magnets are equally spaced around the perimeter of the rotor as shown in the figure. Numeral 4 is an axis of the rotor.
From Fleming's rule, the force F generated by a single turn of the winding for a motor is generally and as a principle derived as follows: EQU F=B.multidot.I.multidot.L
where B is magnetic flux density, I is electric current, and L is effective length of an electric wire which is equivalent to twice the length of the rotor's effective length. The motor power P is represented as follows: EQU P=F.multidot.SP=F.multidot.r.multidot.d.theta./dt
where SP is a peripheral speed of the rotor, r is a radius of the rotor, and .theta. is a rotational angle of the rotor.
Taking voltage here to be V, the electric power P of the motor is represented as follows: EQU P=V.multidot.I=d.phi./dt.multidot.I
where .phi. is flux leakage of a single turn of the winding. Both of the formulas above lead with, P=F.multidot.r.multidot.d.theta./dt=d.phi./dt.multidot.I
As a consequence, torque T generated by the motor is represented as follows: EQU T=F.multidot.r=d.theta./d.phi..multidot.I
That is, the torque T produced by the motor is proportional to the rate of change in rotation d.phi./d.theta. of magnetic flux produced by linkage against the winding.
Accordingly, with reference to the case, for example, of a conventional synchronous motor of bipolar permanent magnet type comprising a stator identical to that shown in FIG. 11 but not being a vernier type motor, which is not illustrated, produced torque T (the rate of change in rotation d.phi./d.theta. of magnetic flux .phi. produced by linkage against winding) is estimated to be simply proportional to magnetic flux B.
The motor shown in FIG. 11 is considered here in the same manner as mentioned above. Suppose that, for example, a single turn of a winding is looped through a slot S5 in the direction from an upper surface to the bottom of the paper, in which FIG. 11 is illustrated, and through S14 in the direction from the bottom to the upper surface of the paper. With respect to the rate of change in rotation of flux leakage .phi. against the winding, represented by d.phi./d.theta..apprxeq..DELTA..phi./.DELTA..theta., in this case, infinitesimal change in rotation .DELTA..theta. is assumed to slightly rotate in a clockwise direction. Infinitesimal change in magnetic flux .DELTA..phi. then appears to increase magnetic flux of the north pole in a direction from the reverse side of the paper to an upper side on eight or nine respective protruding poles of the stator in FIG. 11. Therefore, the rate of change in rotation A.phi./A.theta. of magnetic flux .phi. would show approximately an eightfold increase using simple theory, as compared to the above-mentioned synchronous motor of permanent magnet type, and the same applies to generated torque. As has been stated, a vernier type motor having permanent magnets placed on the rotor is characterized by generation of high torque. However, the effects of limitations of driving frequency and leaked inductance of a motor, in general, make high-speed rotation difficult because the driving frequency for controllable drive increases by approximately eight times.
The motor shown in FIG. 11 has a problem in that effective use has not been made of the magnetic flux of respective permanent magnets. For example, considering magnetic flux generated on the protruding pole of the stator located between the slot S4 and S5, the protruding pole of the stator faces to the north pole of a permanent magnet over a slight gap. This leads to magnetic flux of the north pole appearing on the protruding pole of the stator, which is, at the same time, influenced by many components which close magnetic flux between the north pole and the south pole because of leakage flux from the adjoining pole at each side of the north pole through a nonmagnetic part such as the gap between protruding poles of the stator. The portion of magnetic flux of the north pole which closes the magnetic flux of the south pole is not utilized in a driving operation of the motor. Accordingly, it is impossible to make full use of magnetic flux of the north pole on the protruding pole of the stator located between the slot S4 and S5, and the same on the other respective protruding poles of the stator, which in turn causes insufficient effective magnetic flux to be obtained. As a result, this produces a problem that the motor torque decreases.
The leakage flux of the south pole is explained below through the magnetic properties of a permanent magnet. Marks from B0 to H0 indicated in FIG. 12 show a characteristic example of typical magnetic flux density B and magnetomotive force H of a rare-earth magnet. The operating point of the north pole of a magnet opposing the protruding pole of the stator located between the slot S4 and S5 is OP1. At this point, magnetomotive force H1 mainly functions as the magnetomotive force to an air-gap part and its magnetic flux density is B1. On the other hand, the operating point of the adjoining south poles of each side of the north pole is OP2. At this point, as leakage flux in a nonmagnetic part such as a gap between the slot S4 and S5 is generated, a high magnetomotive force H2 acts and magnetic flux density therein is B2. The portion of B2 closes magnetic flux between adjoining magnets and does not function effectively for a motor. As a consequence, magnetic flux acting in the operation of a motor is that equal to the magnetic flux density of B1 minus B2. Depending on the structure of a motor the value of magnetic flux varies, but the value of B2 may often be more than half of B1 so that half of the power of a permanent magnet can not be utilized in many cases.
Another problem is as follows. Maximum magnetic flux density of magnetic steel in a portion of protruding poles on the stator is as high as 1.7 tesla. Opposed to this, the maximum magnetic flux density in a portion of protruding poles on the rotor is only 1.0 tesla when a rare-earth magnet, whose residual flux density is high, is used and limitations in structure of the rotor interfere with increasing magnetic flux density. Therefore, an increase of motor torque has been expected according to an increase of magnetic flux density in each magnetic pole of the rotor.